To explain a little further (which is probably a euphemism for me showing my ignorance, again)....
If I go to manage My Booking and enter Email Address, Booking Reference and Surname and then click on Forgotten password ? I am asked to enter email address and I will be sent a new password.
But when I press Send I get the message that my email address has not been registered.
So how do I register it.
PS My booking is with Thomson but when I asked them about flights they said to contact Jet 2.